1. Field of the Invention
The invention relates generally to analog-to-digital converter and data transmission circuits for digital processing systems and, in particular, to analog-to-pulse width-to-digital converter and data transmission circuits which use only a single bi-directional data transmission line between sensor locations and the utilization device.
2. Prior Art
One of the problems in the use of digital processors for high volume low cost control systems is the cost and complexity of equipment needed to enter sensor data into a computer. Two problems are associated with combining analog sensors and digital processors. These are: the conversion of the analog output signals into digital form, and the transmission of data from remote sensors to the digital processor. While data from the sensor to the computer may be transmitted in either analog or digital form, the transmission of data in analog form is prone to be noisy therefore special measures must be taken to avoid serious deterioration of the signal. In many instances these measures are costly and not always effective. The transmission of data in digital form not only requires an analog to digital (A/D) converter at each sensor location, but also requires several data lines from each sensor to the computer. Alternatively, a parallel to serial converter/transmitter may be included at each sensor location and the digital data serially transmitted to the digital processor. Neither analog nor digital transmission principles are well suited for an application requiring reliable and low cost equipment, such as for electronic control units having automotive or similar application. An alternative to either analog or digital transmission is the conversion of the analog signal to pulse width signals at the sensor locations and transmitting the pulse width signals to the digital processor where they are converted to digital form as taught by Chi Sun Lai in U.S. Pat. No. 3,801,834 (April 1974) or R. R. Unterberger in U.S. Pat. No. 2,968,022 (January 1961). The transmitted pulse width signal is much less sensitive to the noise problems associated with analog data transmission and only requires a single transmission line between the sensor location and the computer.
In most applications the computer receives input data from several sensor sources. Normally, this data is received serially from the various sensors in response to interrogation signals as taught by Beck et al in Pat. No. 3,059,228 (October 1962). The method taught by Beck et al, however, requires at least one extra lead to each sensor location for interrogation purposes. The requirement for the extra interrogation leads to each sensor location may be eliminated as taught by R. W. Fergus in U.S. Pat. No. 3,678,512 (July 1972). The telemonitoring systems taught by Fergus uses a single bi-directional data transmission line for transmitting interrogation signals and data back and forth between the sensor locations and a digital processor. Each sensor location contains an analog to pulse width converter and a pulse counter which counts interrogation pulses on the bi-directional line and activates the analog to pulse width converter each time a predetermined number of pulses have been counted at each sensor location. The analog to pulse width converter extends the time duration of the interrogation pulse. This extended pulse signal is transmitted back to the digital processor via the bi-directional line. The Beck system also transmits a reset signal to the pulse counters at each sensor location. The reset signal has a polarity opposite the interrogation signal and sets the counter at each sensor location to zero prior to initiating the interrogation pulse sequence.
The disclosed system is an improvement over the prior art discussed above.